In a wavelength division multiplexing (WDM) optical transmission system, optical signals at a plurality of wavelengths are encoded with digital streams of information. These encoded optical signals, or optical channels, are combined together and transmitted through a series of spans of an optical fiber comprising a transmission link of a WDM fiber-optic network. At a receiver end of the transmission link, the optical channels are separated, whereby each optical channel can be detected by an optical receiver.
While propagating through an optical fiber, light tends to lose power. Yet some minimal level of optical channel power is required at the receiver end to decode information that has been encoded in an optical channel at the transmitter end. To boost optical signals propagating in an optical fiber, optical amplifiers are deployed at multiple locations, known as nodes, along the transmission link. The optical amplifiers extend the maximum possible length of the link, in some instances, from a few hundred kilometers to several thousand kilometers, by amplifying optical signals to power levels close to the original levels of optical power at the transmitter end.
An erbium-doped fiber amplifier (EDFA) is one of the most practical types of optical amplifiers employed in many modern fiber-optic networks. A single EDFA module can amplify up to about a hundred of optical channels at a time, thus providing significant cost savings.
It is well known that a bidirectional fiber optical communication system will have about 50% of cost saving, so a bidirectional optical amplifier is highly desired. Bidirectional optical amplifiers are also useful in making an amplifier array commonly known as an arrayed amplifier since a bidirectional amplifiers effectively function as two amplifiers.
Bidirectional optical fiber amplifiers are well known; they pass optical signals therethrough in two opposing or counter-propagating directions. In practice, what makes an EDFA not practicable in many DWDM systems is a very high input power dependent noise figure (NF). In instances where the input power from one direction in a bi-directional amplifier is much higher than the input power from the opposite direction, the NF of the signal with lower input power will be very high and often not acceptable.